Since the discovery of 1α, 25-dihydroxyvitamin D3 (1,25-(OH)2D3), the hormonally active metabolite form of vitamin D3, many analogs have been prepared in order to obtain active compounds with low calcemic affect. Much effort has been expended on modification of the vitamin D side chain. While the 1α hydroxyl group is essential for the hormonal activity, the C-25 hydroxyl group can be replaced with the C-24 hydroxyl group: see, for example, MC 903, (Structure I) or 1, 24-(OH)2 D3 (Structure II).
In the preparation of Vitamin D analogs, a specific stereochemistry for the hydroxyl group at C-24 is necessary for full expression of the biological activity. Under current methodology, the required stereochemistry is introduced by one of three methods: (i) separation of diastereomeric mixture of C-24 hydroxyl epimers via chromatography (see, for example, Calverley, Tetrahedron 4609-4619, 1987); (ii) stereoselective reduction of the corresponding C-24 ketone, (see, for example, U.S. Pat. No. 6,262,283), or (iii) attachment of an enantiopure hydroxyl-carrying side-chain to the Vitamin D skeleton (see, for example, Calverley, Synlett 157-159, 1990).
The stereoselective synthesis is still an unfavorable process for scale up due to its multi-step nature and cost. The chromatographic separation of the epimeric mixture is most widely practiced. The difficulty in chromatographic separation stems from the fact that the two C-24 hydroxyl epimers do not differ greatly in their affinity for the adsorbent, and thus their retention times are too close to allow efficient separation in one chromatographic step. The present invention provides a means to greatly improve the efficacy of chromatographic separation.